The first key element is his knowledge of magic. Harry has learned a great deal from his time at Hogwarts. This knowledge can be applied to create unique magical phenomena in his world. Second, his sense of justice. He might create a world where there is more equality and fairness among wizards and witches. And third, his adventures. The things he has faced in the original story can give him the ideas to make his world either safer or more exciting, depending on his goals.
In fanfic, when Harry Potter creates his own world, his character traits are key elements. His courage, for example, would enable him to take risks in creating a world different from the norm. His empathy would make him consider the needs of all beings in his new world. Additionally, the magical resources available to him are important. Whether it's the Philosopher's Stone or other powerful magical items, they can be used as building blocks for his new world. He also has to consider the existing magical culture and how to either adapt it or break away from it to form his own unique world.
One key element is his magic skills. His ability to perform complex spells and charms would be crucial in building his world. Another is his creativity. Harry has seen a lot in the wizarding world, and he can use that as inspiration to create something new. Also, his relationships play a part. The support of his friends and allies would be necessary for him to bring his vision of a new world to fruition.